There is an increasing need for high heat resistant insulating materials. Poly(m-phenylene isophthalamide) (MPD-I) has proven to be well-suited due to its high heat resistance. However, in forming oil-impregnatable insulating board, the value of the dielectric constant is very important. Mineral oil is considered to have a favorable dielectric constant of about 2.2. The oil-impregnated board typically has a higher value, but provides the mechanical strength needed in the system. The difference in dielectric constants leads to an imbalance in the division of electrical stress between the liquid and solid phases. It would be desirable to redistribute the electrical stress into the board which is more suitable for handling the stress. The dielectric constant of the board should ideally be equal to that of the oil.
A March, 1982 Report of the U.S. Department of Energy, entitled "Advance Concepts for Transformers Pressboard Dielectric Constant and Mechanical Strength" details one study of the problem of the difference in dielectric constants between the board and the oil. The Report discloses Cellulose/MPD-I and Cellulose/PTFE combinations and shows a lowering of the dielectric constant of the board approaching that of oil.
The use of aromatic polyamides and specifically MPD-I in oil-impregnatable insulating boards having high heat resistance is disclosed in U.S. Pat. No. 4,595,457. However, MPD-I pressboard impregnated with mineral oil has a dielectric constant of about 3.4. It would be desirable to have a high heat resistant insulating board having a dielectric constant closer to that of oil.